1. Field of the Invention
This invention relates to a frequency conversion circuit used in a receiving part of wireless telecommunications equipment, especially of cellular telephones or satellite phones.
2. Description of the Related Art
In the field of wireless telecommunications, severe demand to improve the function/performance of a high frequency unit is growing year by year. In a frequency conversion circuit of a receiving part of the high frequency unit, an intense demand to reduce a consumption current has been made especially in order to lengthen the standby time of equipment.
A conventional frequency conversion circuit will be hereinafter described with reference to a drawing.
FIG. 3 shows a conventional frequency conversion circuit disclosed in Japanese Published Unexamined Patent Application No. Hei-6-204751. The frequency conversion circuit in this publication is characterized by being superior in intermodulation distortion and conversion gain, which are important high-frequency characteristics in the frequency conversion circuit.
The frequency conversion circuit of FIG. 3 is comprised of a local oscillator (LO) amplifier 51 for amplifying a LO signal, a mixer 52 for performing a frequency conversion from a radio frequency (RF) signal to an intermediate frequency (IF) signal, and an IF amplifier 53 for amplifying the IF signal that has undergone the frequency conversion.
The LO amplifier 51 is formed of a field effect transistor 54. The mixer 52 is formed of a field effect transistor 55. The IF amplifier 53 is formed of a field effect transistor 56. All sources of the field effect transistors 54, 55, and 56 are grounded.
A gate of the field effect transistor 54 is grounded through a resistor 57 and a resistor 58, and is connected to a LO signal input terminal 60 through a LO input matching circuit 59. A LO signal is input to the LO signal input terminal 60. A drain of the field effect transistor 54 is connected to a voltage supply terminal 62 through a load inductor 61, and is connected to a gate of the field effect transistor 55 through a capacitor 63.
A drain of the field effect transistor 55 is connected to a RF signal input terminal 65 through a RF input matching circuit 64, and is connected to a gate of the field effect transistor 56 through a filter 66.
A drain of the field effect transistor 56 is connected to an IF signal output terminal 68 and a voltage supply terminal 69 through an IF output matching circuit 67.
A cathode of a diode 70 is connected to the gate of the field effect transistor 55, and an anode of the diode 70 is connected to a connection point between the resistors 57 and 58.
The LO input matching circuit 59, the RF input matching circuit 64, and the IF output matching circuit 67 are designed to match characteristic impedance in the frequency where each circuit is used. This characteristic impedance is generally 50 Ω.
Next, the operation of the conventional frequency conversion circuit will be described. As an example, the frequency to be used is set as follows. The RF signal is 2100 MHz, the LO signal is 2300 MHz, and the IF signal is 200 MHz. A power-supply voltage is set to 3V.
In order to operate the frequency conversion circuit, it is necessary to first apply a voltage to the voltage supply terminals 62 and 69. A LO signal that has been input from the LO signal input terminal 60 is amplified by the field effect transistor 54 of the LO amplifier 51, and then is input to the gate of the field effect transistor 55 of the mixer 52 through the capacitor 63. On the other hand, a RF signal that has been input from the RF signal input terminal 65 is input to the drain of the field effect transistor 55.
At this moment, the gate voltage of the field effect transistor 55 is greatly changed by the input LO signal, and, as a result, the channel resistance of the field effect transistor 55 shows weak nonlinearity with respect to time. By the nonlinearity of the channel resistance, the RF signal is subjected to a frequency conversion to an IF signal. The converted IF signal is amplified by the field effect transistor 56 of the IF amplifier 53, and then is output from the IF signal output terminal 68. Since frequency components other than the IF signal are unnecessary, they are removed by the filter 66.
In the present circumstances, this frequency conversion circuit needs to set each current consumed by the LO amplifier 51 and the IF amplifier 53 at 5 mA, in order to satisfy the high-frequency characteristics. Therefore, the frequency conversion circuit consumes a total current of 10 mA.
However, the current value of the LO amplifier 51 and that of the IF amplifier 53 are each set as necessary, at a minimum value by which desired high-frequency characteristics can be achieved. Therefore, the circuit configuration of the conventional frequency conversion circuit cannot greatly reduce the current without lowering the high-frequency characteristics.